WASHINGTON – Secret donors financed more than four out of every 10 television ads that outside groups broadcast this year to influence November’s high-stakes congressional elections, according to a USA TODAY analysis of Kantar Media data.

Two Koch-affiliated groups account for more than one-quarter of the House and Senate advertising from groups that don’t disclose their donors, according to a tally of broadcast ads tracked by Kantar Media’s Campaign Media Analysis Group.

Those Koch advocacy groups, Americans for Prosperity and Concerned Veterans for America, have trained their advertising fire on five Democratic senators up for re-election from red and purple states: Sens. Joe Donnelly of Indiana, Claire McCaskill of Missouri, Jon Tester of Montana, Heidi Heitkamp of North Dakota and Tammy Baldwin of Wisconsin.

In all, nearly 386,000 television spots focused on House and Senate races have aired from Jan. 1 to July 8 this year, ranging from ads by candidates to those funded by outside groups. That total surpasses the 355,464 broadcast TV spots that ran at the same point in the last midterm elections for Congress in 2014 and underscores the battle raging for control of Congress.

Advertising from secret-money groups has jumped, too. A recent analysis by the Wesleyan Media Project and the nonpartisan Center for Responsive Politics found a 26 percent increase in airings by “dark money” groups in federal races since the 2014 midterms.

These organizations generally operate as nonprofit advocacy groups or trade associations that do not have politics as their main purpose. Many of their commercials count as “issue ads” and are not reported to federal election regulators because they do not specifically call for the election or defeat of candidates.

But their growing role means voters know less about the interests trying to influence elections and Congress, said Robert Maguire, who tracks politically active nonprofits at the Center for Responsive Politics.

“We should know if a particular energy bill is going to be a sweetheart deal for an energy donor,” Maguire said.

Outside groups ran nearly 107,000 of the broadcast ads in House and Senate races this year. Organizations identified by USA TODAY as nondisclosing accounted for nearly 47,000 of those ads, nearly 44 percent.

Conservative-leaning groups account for four out of the five biggest secret-money advertisers so far.

Koch officials did not respond to requests for comment about their advertising.

McCaskill, who is running for a third term this year in a state Trump carried by nearly 19 percentage points, has cast herself as a centrist who is willing to work with Republicans. But she joined all of her fellow Democrats in opposing the tax bill Trump signed it into law last December.

In one Americans for Prosperity ad targeting McCaskill’s “no” vote, a woman touts the $2,000 bonus she received from her company as a result of the tax cuts and said McCaskill “let Missouri families down.”

In response, McCaskill spokeswoman Meira Bernstein called the tax package a “windfall” for big business and said “no amount of dark money … will stop Claire from speaking out about it.”

Kantar’s data show about 42 percent of all the advertising in the Missouri Senate race – which pits McCaskill against the Republican state Attorney General Josh Hawley – has come from groups that do not publicly disclose their donors.

The proportion is ever higher in Wisconsin – 46 percent – where Baldwin, a liberal Democrat, is trying to win a second term in a swing state Trump won by less than 1 percent.

The Koch-affiliated Concerned Veterans for America has led the ad blitz against Baldwin, running the most commercials of any outside group in that race, the Kantar tally shows.

The No. 2 outside group active in Baldwin’s race: Vote Vets Action Fund, a liberal group with union ties that does not disclose its donors, either.

“Why are we doing $2.5 million in Wisconsin to help Tammy Baldwin?” Vote Vets chief Jon Soltz said. “Because the Koch brothers are financing a group to attack Tammy Baldwin on veterans’ issues. And we know that Tammy Baldwin has been great on veterans’ issues.”

Soltz said his group complies with the law and operates under the same rules as conservative organizations. And because it does not have the file monthly or quarterly reports with the Federal Election Commission, detailing its contributions and spending, Vote Vets Action Fund can grow its staff, conduct polling and switch strategy "without our opponents seeing how we are spending our money ... or what issues we're working on," he said.

“We use the vehicles that are available to us,” Soltz added. “Right now, the law is the law, and we play within the boundaries of it.”